This paper conducts a study of the history of Chemistry from the
Phlogiston Theory to the Periodic table. It traces the changes and new
discoveries made in Chemistry from the 18th century to the second half of the 19th century. It examines the chemical
revolution of the late 18th century, the discovery of new elements and theories, that eventually
lead to the development of the periodic table. It concludes by an analysis of
the order of the discoveries made in Chemistry during that period. It notes
that many of the discoveries had to occur in a particular order and that the
order of those discoveries was inevitable.

A
new scientific attitude had begun to appear in Europe in the 16th and 17th
centuries but this had little effect on chemistry until the 18th century. In
the 18th century methods for the qualitative and quantitative analysis of
minerals improved resulting in the discovery of new compounds and elements. The
blowpipe became a common laboratory tool while the practice of weighing
precipated salts was introduced by Torben Bergman when analyzing mineral
waters. This practice was improved by Klaproth who heated the salts to dryness
before weighing them, which produced more accurate results. Klaproth also began
the practice of reporting the actual percentage composition as produced by his
analysis regardless of whether it totaled 100% and this allowed the discovery
of errors in analysis and to the discovery of new elements in the materials
analyzed.

The
new laboratory methods lead to the discovery of new elements such as cobalt
(1735), platinum (1740-1741), zinc (1746), nickel (1754), bismuth (1757),
manganese (1774), molybdenum (1781) tellurium (1782), tungsten (1785) and
chromium (1798). The oxides of zirconium, strontium, titanium and yttrium were
also discovered. Many of the new substances were metals and this lead to the demise
of the ancient doctrine of seven metals. In the second half of the 18th century
Carl Scheele discovered hydrofluoric acid and the compounds hydrogen cyanide,
lactic citric and malic acids and glyceral.

The
phlogiston theory was introduced by Becker and Strahl in the late 17th and
early 18th centuries. The theory dealt with mineral creation and considered
there were three different types of earth, terra fluida (mercurious earth),
terra lipidea (vitreous earth) and terra pinguis or phlogiston (fatty earth).
These different types of earth when combined with water would form secondary
principles or substances such as gold and silver. The secondary principles were
extremely stable and could not be broken down into their constituent parts.
Combination amongst the secondary principles would produce other substances
known as mixts, such as metals. Mixts would combine with other mixts to produce
still higher mixts.

All
combustible substances contained phlogiston which was lost to the air during
the process of combustion. A limited amount of air could only absorb a limited
amount of phlogiston which explained why combustion ceased if only a limited
amount of air was available. Combustion would also cease as soon as substances
ran out of phlogiston. The phlogiston released into the air was absorbed by
plants which were eaten by animals so that the phlogiston was recycled into
known combustible materials.

The
main problem with the phlogiston theory is that metals gained weight when burnt
in air and the theory suggested phlogiston was lost, so one would expect
substances to lose weight. This problem became acute when the gaseous state of
matter began to be investigated in the mid 18th century.

It was at this time that gases where becoming
much better understood and progress was made on distinguishing compounds from
elements. In the early and mid 18th century air was considered to be an
element. When scientists observed gases with unique properties, their
difference from air was assumed to be caused by impurities. Boyles inverse
pressure-volume law also convinced scientists that air was an element since the
law applied to all gases. One difficulty with investigating and controlling
gases was solved in the early 18th century by Stephen Hales when he invented an
apparatus for isolating gases so they could be studied separately. Hales device
known as the pneumatic trough allowed the collection of gases above water. A
bent gun barrel, had its closed end containing various substances placed in a
fire and with the open end in a vessel of water suspended upside down in a pale
of water. The gases released from the substances in the close end of the gun
barrel would collect in the upside down container above the water and separate
from the air. Hales apparatus lead to the identification of many gases such as
carbon dioxide discovered by Joseph Black in 1755; hydrogen discovered by Henry
Cavendish in 1766; nitrogen discovered by Daniel Rutherford in 1772; nitrous
oxide discovered by Joseph Priestley in 1772 who in the years after that
discovered ammonia, sulfur oxide and hydrogen chloride; oxygen was discovered
in the 1770's independently by Carl Scheele, Joseph Priestley and Antoine
Lavoisier. The ability to isolate, identify and handle gases soon lead to the
realization that these gases were forms of matter in the same sense that
liquids and solids were.

The
study of isolated gases soon showed they were different from each other and the
differences resulted from differences in composition rather than from
contamination by impurities. The idea of air as an element began to be replaced
by the idea of gas as a state of matter.

The
phlogiston theory was widely accepted by scientists by the middle of the 18th
century. Despite the discovery of oxygen the phlogiston theory continued to be
accepted until Lavoisier created a revolution in chemistry which destroyed the
phlogiston theory, eliminated the four elements of antiquity and replaced them
with the modern concept of elements as substances that could not be broken down
and which were the fundamental substances of chemistry. Lavoisier was also
involved with a reform of the nomenclature of chemistry, so that the names of
compounds reflected the elements making up the compound.

Many
experiments had been conducted by Priestley, Lavoisier and others that showed
that metals and sulfur and phosphorus would increase in weight when burnt in
air. It was also known that when the calx (oxide) produced when these
substances were burnt in air, were themselves burnt using Hale's pneumatic
trough a variety of different airs (gases) would be produced. In particular,
experiments were made involving the burning of mercury in air to produce
mercury calx and then the burning of the mercury calx, using the pneumatic
trough, to recreate the mercury and a gas in which candles burnt more brightly
than in normal air and supported respiration in mice. The air in which the
mercury was burnt was not able to support respiration in mice or combustion
after the formation of the calx. Measurements made indicated that the weight of
the original mercury and air absorbed on burning equaled that of the calx and
also equaled the weight of the mercury and the gas produced when the calx was
burnt.

According
to the phlogiston theory the gain in weight of the mercury as it was burnt in
the air was caused by the release of phlogiston, which had a negative weight.
This explanation was considered by Lavoisier and others as absurd. As the
increase in weight of the mercury equaled the reduction in the air in which the
mercury was burnt Lavoisier concluded part of the air had combined with the
mercury to form the calx. Equally, as the air produced by burning the mercury
calx was different from normal air and as the air left behind when the mercury
was burnt did not support respiration or combustion, it seemed that a
particular constituent part of the air that supported combustion had been
removed from that air and had combined with the mercury to form the calx and
had then been released by the calx into the pneumatic trough. This lead Lavoisier
to assume that air was not an element, but was composed of several parts, one
of which supported combustion and respiration and one that did not. It also
lead Lavoisier to consider that combustion required the presence of the part of
the air that combined with the mercury and did not involve any release of
phlogiston from the substance being burnt. The new gas was eventually called
oxygen, by Lavoisier.

A
further development involved the burning of hydrogen in air which produced a
clear liquid which on analysis was shown to be water. Cavendish, Priestley and
others as well as Lavoisier were involved in these experiments, but Lavoisier
was the first to interpret them to mean that water was a compound of oxygen and
hydrogen. Lavoisier's interpretation marked the end of the belief, that existed
from ancient Greece, that water was an element.

One of the results of the discovery of many
different gases after the invention of the pneumatic trough was the
understanding of gases as a distinct form of matter. Matter could be seen as
changing from a solid to a liquid to a gaseous state by the application of
various degrees of heat.

It
also became clear the air and the gases it was made up of played a role in
chemical reactions. Substances when heated would combine with various gases or
would release gases into the atmosphere. In both cases new substances were
created by these chemical processes.

A
further feature of the discoveries was the confirmation of the law of
conservation of mass. Tacitly assumed by many of the chemists, it was confirmed
by many experiments dealing with gases combining with metals to form calx and
then the calx, when burnt releasing the gases. When the quantity of gas
absorbed by the metal and the amount released from the calx are measured they
are found to be the same confirming the scientists belief in the conservation
of matter.

The
measurement of substances involved in experiments assumed a much greater role
in chemistry, than it had previously. The awareness that metals gained weight
when burnt was an important element in the demise of the phlogiston theory.
Chemistry was becoming a quantitative science. Once this occurred the way was
open for chemical equations and the calculations of the weight of elements
leading to Dalton's atomic theory in chemistry.

Some
of Lavoisier's innovations did not survive. His belief the oxygen was a
necessary part of all acids and his idea of caloric as an explanation of heat
would soon be abandoned. However, by the end of the 18th century, his overall
conception was largely adopted throughout Europe.

A
debate arose between Berthollet and Proust in the early 19th century as to
whether compounds were always formed from fixed proportions of their
constituent elements or whether the proportions could vary. Their debate was
resolved in favor of fixed proportions although there are now known to be some
situations where the constituents of a compound can vary. However in many cases
it became clear that compounds were made up of elements that combined in definite
and fixed proportions. The question arose as to lay behind those definite
proportions. Dalton showed those proportions were not only fixed but related in
a simple numerical manner.

This
process was helped by the development of quantitive analysis in chemistry.
Before the 19th century most work in chemistry was qualitative and concerned
with the properties of substances and the courses of particular chemical
reactions. By the late 18th century more emphasis was being given to the weight
of substances entering into and resulting from chemical reactions.

Lavoisier's
concept of an element provided the foundation for Dalton's atomism. Different
elements had different atoms and this explained the different properties of the
elements. The atomic theory was the outcome of the new quantitative work being
done in chemistry, the discovery of fixed proportions in the elements making up
compounds and the observation that the proportions were fixed in a particular
numerical manner.

The
discovery that air is a mixture made up of a number of gases rather than an
element raised the question of why was it all mixed together rather than formed
in layers with the heaviest gas at the bottom and the lighter gases higher up.
Dalton's answer to this problem was the idea that if the particles of a
particular kind of gas were self repulsive but did not repel particles of a
different kind of gas, then the formation of layers of gases would not occur.
The cause of the repulsion was caloric, Lavoisier's explanation of heat, each
particle of gas being surrounded by an atmosphere of caloric. As heat was known
to flow from hot substances to colder ones, two equally hot substances would be
mutually repellant. The problem remained that all particles of gas had the same
repellant (caloric) wouldn't they still repel each other. Dalton considered
that the particles of different gases were of different sizes and so would have
varying amounts of heat so they would not repel each other. Only particles of
the same gas would have the same amount of heat and would repel each other.
This theory although not correct was the best explanation for the mixing of
gases in air before the kinetic theory of gases was developed in the middle of
the 19th century. However the idea that the size of particles of different
gases would vary, lead to the idea that the weight of the particles would vary.

This
conclusion was also reached as a result of experiments concerning the
solubility of gases in water. It had been observed that the mass of a gas
dissolved by a liquid is proportional to the pressure. Elementary gases such as
hydrogen and oxygen were less soluble while compound gases such as carbon
dioxide were more soluble. Dalton considered the cause of the varying
solubility’s was the different size of the particles of the different gases.
Again the varying size of the particles of different gases lead to the idea
that the weight of the particles would vary.

Dalton
was to call the particles of gases, and of all substances, atoms. Elements were
composed of simple atoms and compounds of compounded atoms. The elements varied
one from the other, as the atoms making up different elements varied in weight.
There were however difficulties in calculating atomic weights. It was impossible
to weigh individual atoms so the system of atomic weights had to be based on a
comparative system. Dalton choose hydrogen as a base for such a system and gave
it an atomic weight of one. The atomic weights of the atoms of other elements
were based on how much more they weighted in comparison with hydrogen. To
calculate for example how much more oxygen weighed than hydrogen Dalton
compared the weight of hydrogenand
oxygen making up a quantity of water. He found the oxygen in water weighed 5.5
times as much as the hydrogen (the correct figure is 8) so he assigned an
atomic weight of 5.5 to oxygen. Such a system would only work if the number of
hydrogen and oxygen atoms in water was known, and in Dalton's time this was not
known. To overcome this difficulty Dalton adopted his principle of simplicity
when he assumed that if two elements formed only one compound, the compound
would consist of one atom of each element. If there were two compounds formed
of the same two elements, there would be two atoms of one element and one atom
of the other element and so on. As water was the only known compound of
hydrogen and oxygen it was assumed to consist of one hydrogen and one oxygen
atom. Obviously the principle of simplicity was not a reliable guide to the
chemical composition of compounds. The problem of accurate measurement of
atomic weights and of accurately assessing the chemical make up of compounds
limited the usefulness and acceptance of the atomic theory. In addition as the
number of elements discovered increased in the early 19th century, it began to
look as though there was an increasing number of fundamental particles. Many
scientists considered the idea that there were a large number of fundamental
particles was absurd.

The
atomic theory did obtain support from Gay-Lussac in 1808. Gay-Lussac found that
hydrogen combined with oxygen at a ratio of approximately two to one. In other
experiments he discovered other gases combine among themselves in simple whole
number ratios. This became known as the law of combination of gases. It
suggested equal volumes of different gases contained the same number of
particles.

However
there were problems with the law of combination of gases by volume. Carbon
monoxide, considered to contain one atom of oxygen and one atom of carbon
should be denser than oxygen. Yet, it was known to be less dense than oxygen. A
further problem was that one volume of nitrogen combined with one volume of
oxygen to give two volumes of nitric oxide rather than the one compound of
nitric oxide.

A
resolution to these problems was offered by Amadeo Avogadro. He distinguished
between an atom, as the smallest part of an element which can play a role in a
chemical reaction and a molecule as the smallest part of a substance. He
assumed that molecules of an element could consist of more than one atom of the
element. This meant a molecule of hydrogen could contain two atoms of hydrogen.
This also meant a molecule could split in two when involved in a chemical
reaction. If this happened, then equal volumes of gases could contain the same
number of particles. The reaction of nitrogen and oxygen could be explained by
two atoms of oxygen joining two atoms of nitrogen to create two molecules of
nitric oxide.

However
Avogadro's theory was largely ignored. The terminology he used to explain his
theory was difficult and many chemists refused to accept that the fundamental
particles of elements could contain more than one atom. Avogadro's theory was
only adopted after 1860 when Cannizzaro drew chemists attention to it and
explained how it could allow the correct calculation of molecular and atomic
weights.

It
was only after 1860 that the atomic theory gained considerable acceptance with
the acceptance of Avogadro's theory which cleared up problems concerning the
atomic weight of elements and the composition of compounds. This was followed
by Mendeleev's periodic table and finally Einstein's 1905 explanation of
Brownian motion as grain pollens being bumped about by the movement of atoms
confirmed the atomic theory.

A
new method of causing chemical decomposition became available around 1800. This
involved the voltaic pile which allowed a continuous electric current to be
passed through a substance causing decomposition. Before 1800 only static
electricity had been available for chemical decomposition but the short term
nature of the current limited its effectiveness in chemical experiments. The
use of the voltaic pile was to allow the discovery of new elements and showed
some substances previously considered to be elements, were actually compounds.
Sir Humphry Davy was to isolate potassium, sodium, barium, strontium and
magnesium by means of the voltaic pile while Gay-Lussac and Thenard discovered
boron and Courtois discovered iodine. The isolation of potassium lead to the discovery
of other elements due to the chemical reactivity of potassium. The heating of
various compounds with potassium resulted in the discovery of silicon and
aluminum.

The
use of the voltaic pile lead to a further significant discovery. It was
observed that when water was decomposed using the voltaic pile, the hydrogen
and oxygen formed at different poles. It was then found that when an electric
current was passed through solutions of salts, acids formed at the positive
pole and bases at the negative pole. This observation lead to the development
of Berzelius's dualistic theory. Berzelius considered that atoms carried both a
positive and negative charge, but only one of the charges was predominant.
Metals were electro-positive as they were attracted to the negative pole in
electrolysis. Oxygen was the most electro-negative element. As atoms possessed
both charges an atom could be negative towards one element and positive towards
another. Phosphorus for example was negative towards metals, but positive towards
oxygen. This allows a series to be established from the most electro-positive
element to the most electro-negative element.

Chemical
combination happened due to the attraction between opposite electrical charges.
When such combination occurred, the compound formed would be either positive or
negative depending on the strength of the charges of the elements making up the
compound. If the compound was positive it could then combine with negative
compounds and elements and vice versa. Berzelius considered his theory
explained the nature of chemical affinity. It would not however be accepted
today.

In
the early 19th century chemists began to make a distinction between organic
chemistry, which concerned materials obtained from animal and vegetable
sources, and inorganic chemistry which dealt with materials from other sources.
As the knowledge of organic materials increased organic chemistry became
synonymous with the chemistry of carbon compounds.

Organic
analysis at the start of the 19th century was only capable of separating
mixtures of related substances and often these procedures resulted in
significant chemical alteration of the substances resulting in misleading
results. Lavoisier was the first to develop improved analytic methods for the
study of the carbon and hydrogen content of organic materials. He burnt the
materials in oxygen or air and weighed the carbon dioxide and water that was
formed. Gay-Lussac and Thenard improved the method by reacting the organic
material with the oxidizing agent potassium chlorate. The method was further
improved when copper oxide replaced the potassium chlorate. The method was
further improved by Berzelius and in 1831 by von Liebig. Liebig's method
allowed reliable analysis to take place and was to survive into the twentieth
century. Methods for the determination of nitrogen in organic substances were
devised by Dumas and for the determination of sulphur and halogens by Liebig.

In
the early 19th century most chemists believed the products of a living organism
were produced through the agency of a vital force present only in living plants
and animals. These products could be converted into other products in the
laboratory, but could not be created in the laboratory from their elements.
This view known as vitalism received a serious blow from Wohler in 1828 when he
synthesized urea by reacting silver cyanate with ammonium chloride. This
however did not mean the end of vitalism because although Wohler had created an
organic product in the laboratory, he had done so by the reaction of two other
organic products. Vitalism received a major set back in 1844 when Kolbe
synthesized acetic acid from non-organic materials and Berthelot in 1860 showed
the possibility of the organic synthesis of organic compounds from the elements
carbon, hydrogen, oxygen and nitrogen leading to the abandonment of vitalism.
The development of the concept of the conservation of energy in the middle of
the 19th century showed there was no need for the concept of a vital force.

The
first attempt to understand the nature of organic compounds was the theory of
radicals. Lavoisier considered that when a radical combined with oxygen an acid
was formed. The radical was an element for mineral acids, but was a compound
containing both carbon and hydrogen for organic acids. The radicals of
different organic acids contained different quantities of carbon and hydrogen.
The idea of the radical was extended by experiments by Gay-Lussac on hydrogen
cyanide and cyanogen. The cyanide radical was observed to pass unchanged
through a series of reactions so radicals came to be seen as a particularly
stable group of atoms that reacted as a unit during chemical reactions.

Support
for the radical theory came from the work of Liebig and Wohler in 1832 on
benzaldehyde. They converted it to a number of other compounds including
benzoyl chloride and benzoic acid and throughout all the changes the group C14 H10 O2 (actually C7
H5 O) remained unchanged. They called this
the benzoyl radical. Radicals soon became known as the elements of organic
chemistry and chemists began to look for radicals in every compound.

Radical
theory however was modified when Dumas studied the reaction of chlorine,
bromine and iodine with oil of turpentine and other substances. He considered
halogens, such as chlorine, bromine and iodine, were replacing hydrogen within
compounds and a given volume of hydrogen was being replaced by the same volume
of halogens. He called this discovery the law of substitution. Dumas work was
taken up by Laurent who considered hydrocarbons to be a "fundamental
radical" by which "derived radicals" could be obtained by
substitution reactions. Derived radicals had similar properties to the fundamental
radicals they replaced. Laurents's theory meant radicals could no longer be
considered to be an unchangeable group of atoms.

Laurents
theory was attacked by Berzelius as it conflicted with his dualistic theory.
Berzelius could not see how electro-negative chlorine could replace
electro-positive hydrogen without totally changing the compound. However the
experimental evidence, in particular the involving the chlorination of acetic
acid and the studies of the properties of the resulting trichloraacetic acid by
Dumas and Melsens lead to an abandonment of the dualistic theory as it applied
to organic compounds.

The
dualistic theory was opposed by a number of unitary theories proposed by
Laurent, Dumas and Gerhardt. Laurent's theory was known as the "nucleus
theory", Dumas as the theory of types and Gerhardt's as the theory of
residues. The proliferation of theories reflected the confusion prevailing in
organic chemistry in the first half of the 19th century.

The
concept of radicals was enhanced by Bunsen who studied the reactions of a
liquid known as cacodyl. He treated cacodyl chloride with zinc and obtained
what he considered to be the free cacodyl radical. Later Kolbe isolated what he
considered to be the free methyl radical (actually it was ethane) while
Frankland believed he had isolated the free ethyl radical (actually butane) by
treating ethyl iodine with zinc.

A
new type theory was then developed due to the work of Wurtz, Hoffman,
Williamson and Gerhardt. Wurtz discovered the primary amines, methyl and ethyl
amide. He recognized these compounds were derivates of ammonia, in which
hydrogen had been replaced by methyl or ethyl. Hoffman showed the relationship
of the amines to ammonia by alternatively replacing one, then two then three
hydrogen atoms by organic radicals to create primary, secondary and tertiary
amines. Hoffman considered all these compounds to belong to an "ammonia
type".

Williamson
prepared ether by the action of ethyl iodine on potassium ethlate. This meant
that a hydrate theory of the structure of alcohols proposed by Liebig was
incorrect and Williamson suggested the concept of a water type. This type
included compounds such as water, alcohol, ether and methylethyl ether and
acetic acids and other acids of that series.

Gerhardt
added two further types. These were the hydrogen type that included hydrogen,
ethane and butane and the hydrogen chloride type which included hydrogen
chloride and ethyl chloride. Gerhardt considered that organic compounds could
all be classified as belonging to one of the four types, being the water,
hydrogen, hydrogen chloride and ammonia types.

The
new type theory was useful as a means of classifying the increasing number of
organic compounds but it did little to explain the constitution of the
compounds or the arrangement of the atoms in the molecule. The chemical
formulae according to type theory were reaction formulae. They represented
methods of formulation and decomposition and indicated there were chemical
similarities between compounds of the same type. To understand the constitution
of the compounds it was necessary to go beyond the radicals to the atoms and to
look at the arrangement of the atoms in the radical.

Progress
to understanding the constitution of compounds and radicals was made by
Frankland. His studies of the combination of organic materials with metals and
other experimental work showed that an atom of nitrogen, phosphorus, antimony
and arsenic always combined with three or five organic radicals. Mercury,
oxygen and zinc combined with two. Frankland considered "no matter what
the character of the uniting atoms may be, the combining power of the
attracting element ... is always satisfied by the same number of these
atoms". The term combining power was eventually replaced by valency. The
idea of valency was supported by the law of definite and multiple proportions
which implied that atoms had an exact and limited capacity to combine. Valency
was also suggested by the experimental observation of substitution. It was
known since 1834 that one atom of chlorine replaced one atom of hydrogen while
one atom of oxygen replaces two atoms of hydrogen. Frankland's theory was not
immediately accepted because formulae were uncertain and based upon uncertain
atomic weights and because chemists were still imbued with the concepts of
radicals and types.

Frankland's
work was carried on by Kekule and Couper who simultaneously in 1858 produced
improved theories of chemical structure. Their theories brought clarification
to ideas that were slowly developing amongst chemists over the previous twenty
years. Their ideas had two main themes, the quadrivalency of carbon and the
ability of carbon atoms to join together to form a carbon chain. The linking
together of carbon atoms explained the formation of organic compounds
containing many carbon atoms. Their work also showed how the linking together
of the atoms of a compound could be shown diagrammatically. The quadrivalency
of the carbon atom was later abandoned when subsequent research revealed that
in some cases carbon is divalent and trivalent.

The
work of Kekule and Couper provided an explanation for the structure of
aliphatic compounds as consisting of chains of carbon atoms, but it did not
explain aromatic compounds which always contained at least six carbon atoms in
the molecule. The simplest of the aromatic compounds was benzene, which had
been discovered by Faraday in 1825. Kekule suggested the structure of the
benzene molecule was a closed ring of six carbon atoms, each of which had a
hydrogen atom attached to it. Kekule theory for the benzene molecule and the
nature of aromatic compounds was later to receive experimental confirmation.

The
idea of the linking of atoms and the structural representations that were
derived from it form the basis of modern organic chemistry. These theories
enabled chemists to understand the relationships between already known
compounds and to discover and create a vast number of new compounds.

The
later half of the 19th century saw many more developments in structural
chemistry. In 1862 a triple bond was discovered for acetylene and in 1864 a
double bond for ethylene. In 1870 Markovnikov's rule was formulated concerning
the influence of neighboring groups on the reactivity of individual parts of
organic molecules. Van Hoff and Le Bel provided a picture of the valence bond
as uniformly directed in space. The three dimensional view of valence lead to
the idea of new types of isomerism, such as cis-trans isomerism.

One
major problem for chemistry in the mid 19th century was confusion over atomic
weights, molecular weights and equivalents. This confusion made it impossible
to write chemical formula with confidence. In a chemistry book written by
Kekule he quoted nineteen different formulas that had been suggested for acetic
acid. Atomic weights in the first half of the 19th century were decided by
guess work and arbitrary rules. Gerhardt corrected Berzelius's atomic weight
for sodium and silver by halving them, but he also halved the correct weights
for zinc and calcium and so made them incorrect.

An
attempt was made to clear up the confusion by Cannizzaro in an article
published in 1858. In this article Cannizzaro advocated the use of Avogadro's
theory and explained the misunderstanding that had prevented the earlier
acceptance of the theory. He also explained how the theory allowed the accurate
calculation of molecular and atomic weights.

Avogadro's
theory provides for the calculation of molecular weights of substances in the
gas or vapor state by determinations of the gas or vapor densities. The
determination of densities is made on a relative basis with hydrogen, as the
lightest gas, being chosen as the standard. All other substances are expressed
as having a weight relative to that of hydrogen. However as the hydrogen
molecule consists of two atoms, atomic weights should be related to the weight
of half a molecule of hydrogen. Cannizzaro's ideas supporting Avogadro's theory
provided clear information about the number of atoms in a molecule of a
compound and provided a firm foundation for the writing of chemical formula.

An
important new development occurred in 1860 with the development of the
spectroscope which allowed the detection of new elements. The color spectrum
had been known in Roman times and the refraction of light had been studied by
the Arabs, Roger Bacon, Kepler and Descartes. Newton showed that a prism
separated white light into its component colors and that another prism could
turn the separated colors into white light. The spectrum was extended into the
infrared in 1800 by William Herschel and into the ultraviolet in1801 by W H
Williamson.

Dark
lines in the spectrum of sunlight passed through a prism were observed by
Joseph Fraunhofer in 1814. He studied and mapped the lines (eventually called
Fraunhofer lines) and observed similar lines in the spectrum of light from the
moon, planets and stars. Fraunhofer also discovered yellow lines in the
spectrum of the flame he was using when studying the refractive index of
samples of glass. Similar lines were observed in the flame of burning alcohol,
oil and tallow when determining refractive indices. Such lines had also been
observed in the spectra of many substances by many scientists. They had been
observed in the spectra of metallic salts by Thomas Melvill in 1752. David Brewster
had observed them in the spectra of "nitrous acid gas", sulphur, and
iodine vapor and brown oxide of nitrogen. Similar studies were carried out on
halogen vapors and other gases.

The
identification of substances by means of the spectrum began when Andreas
Marggrat used flame colors to distinguish sodium and potassium salt in 1758.
John Herschel showed when the flame colors of boric acid and the chlorides of
barium, calcium, strontium and copper were passed through a prism they showed
certain lines which could be used to identify the substances. Brewster after
observing sulphur vapor absorbed light from the violet end of the spectrum and
iodine vapor absorbed it from the middle part, suggested "the discovery of
a general principle of chemical analysis in which simple and compound bodies
might be characterized by their action on definite parts of the spectrum".

This
idea was put into practice by the invention of the spectroscope by Bunsen and
Kirchoff in 1859. Bunsen used flame colors for the identification of salts in
mineral water. Kirchoff suggested better results could be obtained if the light
was passed through a glass prism and viewed as a spectrum. Kirchoff also
outlined the reason for the bright and dark lines as being emission or
absorption lines of light. He set out his laws of spectroscopy as

1. An incandescent body gives off
a continuous spectrum.

2. An excited body gives of a
bright-line spectrum.

3. White light passed through a
vapor has dark lines where the vapor ordinarily emits light.

The spectroscope provided
chemists with an instrument of unprecedented sensitivity for the analysis of
chemical substances. The spectroscope was able to map Fraunhofer lines with
great accuracy and when only minute traces of an element were present.

The
effects of spectroscopy soon became apparent. Bunsen and Kirchoff discovered
cesium in 1860 and rubidium in 1861. Thallium was discovered in 1861 by Crookes
and indium in 1863 by Reich and Richter. Spectroscopy was later involved in the
discovery of gallium, the rare earths and the rare gases.

When
Lavoisier provided the modern definition of an element as a substance that
could not be broken down into simpler substances, he provided a list of 33
elements. These included several forms of energy and some substances later
found to be compounds. However the attention Lavoisier drew to the elements and
new analytical techniques such as the voltaic pile and reaction with potassium,
once that element had been isolated by Davy, lead to the discovery of many
additional elements. Between 1790 and 1844 31 new elements were discovered but
the number of elements remained limited to 58 from 1844 to 1860 as the unknown
elements were generally present in minerals in to small quantities to be
detected by the analytical techniques available at that time. It took the
development of the spectroscope to allow the discovery of new elements to
recommence.

The
discovery of sufficient elements and the establishment of a reliable system of
calculating atomic weights as provided by Avogadro were necessary before the
next important development in chemistry. This was the system of classification
of the elements known as the periodic table.

The
first attempt at such a classification was made by Dobereiner in 1829.
Dobereiner observed that it was possible to put the elements into groups of
three with the atomic weight of one element in the group being the mathematical
average of the other two elements. Dobereiner also observed that the members of
the groups all had similar chemical properties.

A
number of other attempts were made to discover some sort of relationship
between the elements. The more significant of these were made by Beguyen de
Chancourtis and John Newlands. Beguyen de Chancourtis in 1862 and 1863 with the
benefit of the atomic weights that were accepted after the work of Cannizzaro
persuaded chemists to accept Avogado theory, arranged the elements in order of
their increasing atomic weights around a cylinder. Beguyen de Chancourtis
pointed out there were remarkable similarities in the elements on the same
vertical line on the cylinder.

Newlands,
also using Cannizzaro's atomic weights, arranged the elements in order of their
atomic weights. He observed that similar elements would appear on a horizontal
line if a new column was commenced with each eight element. In some versions of
his table Newlands used blank spaces for unknown elements, but in other
versions he did not. Newlands called his arrangement "the law of
octaves" but as with Beguyan de Chancourtis, Newlands work was largely
ignored.

The
idea of the periodic table was accepted due to the work of Mendeleev and Meyer.
Mendeleev published his first periodic table in 1869 and a second version was
published in 1871. Mendeleev arranged the elements in the order of increasing
atomic weight and noted the properties of the elements recurred periodically in
the table. Gaps were left in the table where Mendeleev considered there were
elements yet to be discovered. Using his periodic table Mendeleev predicted the
discovery of certain new elements. He stated the approximate atomic weights,
valences, the sorts of compounds the element would be found in and other
properties the undiscovered elements would have. When the new elements gallium,
scandium and germanium were discovered and found to have properties extremely
close to those predicted by Mendeleev, his periodic table became widely
accepted.

Meyer's
periodic table was broadly similar to Mendeleev's but tended to concentrate on
the physical properties of the elements, while Mendeleev's concentrated on the
chemical properties. Meyer produced a graph in which he plotted the atomic
weights against atomic volumes of the elements. The graph showed similar
elements appeared at similar places on the curve produced by the graph. Alkali
metals appeared at the high points of the curve; non metals on the ascending
sides and metals on the descending sides and in the low points on the curve.
Meyer's graph helped to make the periodicity explicable and encouraged
acceptance of the periodic table.

Comment

The
brief foregoing history of chemistry up to the development of the periodic
table is provided to show certain aspects of the theory suggested in the paper A Theory of History. One of those aspects
concern human needs. The needs meet by human kinds exploration of the nature of
matter was simply the desire to know and understand the universe. Such needs
seem to exist in all societies as for example all societies seem to have
creation myths to explain how the universe came into existence. Equally many
societies have their own explanations as to the nature of matter. The
Babylonians considered all matter was created from a first principle and that
first principle was water. Such needs to explain the nature of matter would be
covered by Maslows cognitive needs, the need to know, to understand and to
explain

The foregoing history of chemistry is also
intended to show how the discoveries made in chemistry were made in a
particular order and had to be made in that order. One such discovery was
Lavoisier's chemical revolution. This had a number of features such as elements
being seen as substances that could not be broken down; air no longer seen as
an element; air playing a role in chemical reactions; the concept of a gas
being seen as a separate state of matter; confirmation of the law of
conservation of mass; explanations of combustion and respiration and a theory
of acidity. It seems very apparent that many features of this revolution were
dependant upon the discovery of new gases in the mid and late 18th century and
in particular upon the discovery of oxygen. It is hardly conceivable that
Lavoisier's explanation for combustion and respiration could have been made
without the prior discovery of oxygen. Lavoisier's experiments with mercury
calx showed that part of the air, the part we call oxygen could cause candles
to burn more brightly, but that air without oxygen could not support combustion
or respiration in mice. In order to make these discoveries it was necessary for
oxygen to be isolated and identified as a particular component of air. Only
then could its effect on combustion and respiration be studied and understood.
The discovery of oxygen was itself dependent upon a means of isolating and
controlling gases as was provided by the pneumatic trough. Without the
pneumatic trough Lavoisier would have only been able to perform part of his
experiment with mercury calx. He could have burnt the mercury in air to create
the calx an experiment that had been preformed many times before. But without
the pneumatic trough he could not have burnt the calx in a situation where the
oxygen released by the combustion could be controlled and experimented with.
The same or similar experiments had been preformed by Priestly and Scheele but
Lavoisier was the first to interpret the results with a new theory of
combustion and respiration. Furthermore, Priestly and Scheele were as dependent
on the pneumatic trough as Lavoisier was when it came to the discovery of
oxygen. The consequence is that there was an order of discovery from the
pneumatic trough to oxygen to Lavoisier's theory of combustion and respiration.

A
similar order of discovery was involved in the discovery that air was not an
element, that the gases in the air played a role in chemical reactions and the
concept of a gas as a separate state of matter. The discovery of many gases
using the pneumatic trough showed the air was made up of a number of gases and
so was not an element. The role of gases or air in chemical reactions was shown
by the gain in weight of metals when burnt in air to produce a calx and then
the release of gases when the calx was burnt. The concept of a gas as a
separate state of matter was shown when substances could be heated and could be
shown to pass through solid, liquid and gaseous states. Again the pneumatic
trough played a part in this as substances could be burnt and could be shown to
produce particular gases. There was an order of discovery from the pneumatic
trough to the discovery of gases, to the conclusion that air is not an element,
that it plays a role in chemical reactions and that gases are a separate state
of matter.

The
proof of the law of conservation of matter was dependant upon both the
pneumatic trough and upon the increasing use of quantitative studies in
chemistry. When metals were burnt, it was known, before Lavoisier, that they
increased in weight. However it was only with the use of the pneumatic trough
that it was possible to isolate air to show that there was a decrease in the
weight of the air that matched the increase in the weight of the metal. The
accurate measurement of the decrease in the weight of the air and the increase
in the weight of the metal were required before the law of conservation of mass
could be confirmed.

The
modern concept of an element as a substance that could not be broken down, was
established by Lavoisier, when the traditional elements such as air and water
were shown to be made up of simpler substances. Due to the use of the pneumatic
trough and the discovery of a number of gases in the mid and late 18th century
Lavoisier was able to provide a list of 33 elements, which replaced the
traditional elements of earth, fire, air and water. This involved an order of
discovery from the pneumatic trough, to the gases discovered through the use of
the pneumatic trough to Lavoisier's new concept of elements.

Lavoisier's
theories of acids and caloric, while no longer considered correct, were
dependant upon the prior discovery of oxygen, which itself was dependant upon
the prior discovery of the pneumatic trough. This meant an order of discovery
from the pneumatic trough to oxygen to Lavoisier's theories of acid and
caloric.

The
next major development in chemistry, after Lavoisier's revolution, was Dalton's
atomic theory. The atomic theory was based upon Lavoisier's concept of an
element. A particular element would consist of a particular atom and different
elements had different atoms with different weights. It was improvements in
quantitative chemistry in the late 18th century, that showed that different
elements had different atomic weights. The law of definite and fixed
proportions which suggested compounds were made up of elements in definite and
fixed proportions and the proportions were related in a simple numerical way
provided support for the atomic theory. This was because the atomic theory
explained the relationship between the elements as varying in weight in that
simple numerical manner. The atomic theory was also based on Dalton's erroneous
ideas as to why the different gases in the air did not form layers. Such ideas
naturally could only be formed after it had been discovered that the air was
made up of a number of different gases. The discovery that air was a mixture
was dependent upon the experiment by Lavioisier and others which isolated
oxygen, nitrogen and other gases which were dependent on the prior discovery of
the pneumatic trough. This means a chain of discoveries runs from the pneumatic
trough, to the isolation of oxygen and nitrogen, to the idea of air as a
mixture. A further factor in the development of the atomic theory were
experiments concerning the solubility of gases in water. These again were
dependant on the isolation of various gases by the pneumatic trough, so an
order of discovery from the pneumatic trough to the isolation of various gases
to the atomic theory can be identified.

The
atomic theory could, at the time of Dalton, and eventually did in the 1860's
receive support from Gay-Lussac's law of combination of gases which suggests
equal volumes of different gases contained the same number of particles. Such a
law could only have been proposed when a number of gases had been isolated
which required the prior discovery of the pneumatic trough. Yet again an order
of discovery can be identified from the pneumatic trough to the discovery of
gases, to the law of combination of gases to the atomic theory.

The law of combination of gases
naturally lead to Avogadro's theory. Avogradro's theory was intended to explain
the known behavior of gases during chemical reactions in a manner consistent
with the law of combination of gases. This means both the experiments showing
the chemical reactions and law of combination of gases were necessarily prior
to Avogadro's theory. The acceptance of Avogadro's theory in the 1860's lead to
an accurate system for calculating atomic weights and to the eventual
acceptance of the atomic theory.

The
discovery of the voltaic pile lead to the discovery of new elements, including
potassium, and the discovery of potassium lead to the discovery of further
elements. The voltaic pile also lead to the discovery of the electro-positive
to the electro-negative series which in turn lead to the development of the
dualistic theory. A clear order of discovery runs from the voltaic pile to the
discovery of new elements and to the dualistic theory.

Organic
chemistry began with the development of new and improved methods for organic
analysis. The new methods experimental results showing groups of atoms which
passed unchanged through a series of reactions lead to the development of the
radical theory. The radical theory was forced to be modified when new
experimental results showed halogens replacing hydrogen with organic compounds
leading to the law of substitution. The law of substitution spelt the end of
the dualistic theory in relation to organic compounds and gave rise to a series
of theories by Laurent, Gerhardt and Dumas. Eventually the new type theory
arose as a means of classifying organic compounds. There is an order of
discovery from improved analytic methods, to new experimental discoveries which
then resulted in new theories to explain the experimental results. This process
occurred with both the radical theory and the development of the law of
substitution and the theories created by Laurent, Gerhardt and Dumas which
attempted to explain the law of substitution. The development of the new type
theory also resulted from experimental results that showed that certain
compounds were related to each other.

The
same situation is shown by the efforts of chemists to understand the structure
of compounds and radicals. Frankland's study of the combining of organic
materials with metals was dependent upon the improved analytic techniques
brought into organic chemistry by von Liebig. Frankland's experimental work
lead to the development of the concept of combining power or valence.
Frankland's work was carried on by Kekule and Couper who produced theories
concerning the quadrivalency of carbon and the linking together of carbon atoms
to form carbon chains. This then lead to Kekule's theory of the structure of
the benzene molecule and of aromatic compounds. Again one sees new analytic
methods leading to experimental results, which then lead to the concept of
valency, which then lead to theories concerning the valency of carbon and to
the structure of the benzene molecule. There was a specific order of discovery.

The
development of the periodic table was dependent upon certain prior discoveries.
One was an accurate way of calculating atomic weights, another was the
discovery of a sufficient number of elements to allow them to be organized in a
coherent table and a further development was sufficient analysis of the properties
of the elements to reveal the periodicity of the periodic table. A reliable
method of calculating atomic weights was provided by Avogadro's theory. The
discovery of the elements occurred throughout the 18th and 19th century
assisted by new instruments for the investigation of chemical substances such
as the pneumatic trough, the voltaic pile, potassium analysis and the
spectroscope. It probably took the invention of the spectroscope in 1859 to
allow the discovery of sufficient elements so a coherent periodic table could
be created. The spectroscope itself could only be invented after the discovery
of the spectrum, Fraunhofer lines and that Fraunhofer lines allowed the
identification of chemical substances. Only when a sufficient number of
elements had been discovered and their properties analyzed was it possible to
create the periodic table. Early attempts to create a coherent table of the
elements by Dobereiner, Beguyen de Chancourtis, John Newlands and others had
failed due to uncertain atomic weights and an insufficient number of elements
being known. It was in 1869 that Mendeleev published his periodic table and
1870 when Meyer published his.

It
is of course not surprising there is a specific order of discovery in chemistry
from Lavoisier's revolution to the periodic table. Obviously the facts of
chemistry were not all discovered at the same time. However it is contended
that the order in which the discoveries were made were, in many cases,
inevitable and the discoveries in those cases could not have been made in any
other order. In some cases this is obviously so, a good example being the use
of the spectroscope to identify elements. It is simply not possible to invent
the spectroscope without first discovering the spectrum, then discovering
Fraunhofer lines within the spectrum and then the discovery that Fraunhofer
lines showed the presence of particular chemical substances. An awareness of
all these discoveries was a necessary ingredient to the invention of the
spectroscope and all these discoveries had
to take place in the order in which they did take place.

A
further example would be that the development of the periodic table which
required certain prior discoveries. The first was the establishment of a
coherent definition of an element, distinguishing elements from compounds and
mixtures. It was then necessary to create a list of elements and to study their
properties, including their atomic weights. Only when a reasonable number of
elements had been discovered and their properties had been assessed with a
reasonable degree of accuracy was it possible to understand the relationship
between the elements and so produce the periodic table.

Yet
another example would be Lavoisier's theory of combustion and respiration. It
would not have been possible for Lavoisier to come up with such theories
without the prior discovery of oxygen.

However
other discoveries will take place in a certain order without the later
discoveries being dependent upon the earlier discoveries. The dualistic theory
was necessarily dependant upon the prior discovery of the electro-positive and
electro-negative series and the voltaic pile. It was not however dependant on
Lavoisier's chemical revolution and yet it occurred after the chemical
revolution. Could the dualistic theory have been created before the chemical
revolution? It is possible the dualistic theory could have been invented before
the chemical revolution. However such an event was unlikely because discoveries
always vary in difficulty; some discoveries will be easier to make than others.
The easier discoveries will tend to be made earlier than the more difficult
discoveries. This will not always be the case if for example governments and
corporations pour resources into a particular area and neglect other areas.
Difficult discoveries in areas receiving the resources may well be made before
easier discoveries in areas not receiving the resources. The invention of the
atomic bomb during World War II and the discoveries made in the space race are
such accelerated discoveries caused by governments pushing resources into
particular areas. However such situations are unusual and normally easier
discoveries will be made before later ones.

A
further such example would be that Lavoisier's chemical revolution occurred
over a hundred years later than Newton's establishment of classical physics.
Could Lavoisier's revolution have occurred before Newtons? As Lavoisier's
revolution was not dependent upon the earlier revolution in physics it would
have been possible for it to have occurred before Newtonian physics. However
such a situation would be very unlikely if the discoveries required for
Newton's revolution were easier than those required for Lavoisier's. This would
seem to be the case as Newton's revolution was substantially dependable on
directly observable phenomena, the only exception being an accurate
understanding of planetary orbits which required the telescope. Lavoisier's
revolution was dependant upon phenomena that could not be directly observed
such as gases that were discovered by the use of the pneumatic trough. If
however, the pneumatic trough had been invented earlier, the Chemical
revolution could have occurred earlier.

The
final result is that there were certain discoveries in Chemistry which could
not have taken place without certain prior discoveries. These were cases in
which the order of discovery was inevitable and no other order of discovery was
possible. There were other cases where the order of discovery was not
inevitable, but where there was a likely or probable order of discovery as
certain discoveries were easier to make, than other discoveries, and so were
likely to be made earlier than the other discoveries. The degree to which one
discovery was easier than another would determine the likelihood of it being made
before the other discovery.